Enhanced random access channel (rach) procedure

ABSTRACT

The present disclosure describes a method, an apparatus, and a computer-readable medium for a random access channel (RACH) procedure at a user equipment. For example, the method may select a two-step RACH procedure or a four-step RACH procedure at the UE based at least on RACH configuration information received from a base station or the RACH configuration information at the UE. The example method may further include transmitting, from the UE, one or more messages associated with the two-step RACH procedure or the four-step RACH procedure based on the selection.

CLAIM OF PRIORITY

The present application for patent is a Continuation of U.S. patentapplication Ser. No. 15/623,001, filed on Jun. 14, 2017, entitled,“Enhanced Random Access Channel (RACH) Procedure which claims priorityto U.S. Provisional Patent Application Ser. No. 62/410,168, filed Oct.19, 2016, entitled “Enhanced Random Access Channel (RACH) Procedure,”each of which is assigned to the assignee hereof, and are herebyexpressly incorporated by reference herein in their entireties.

BACKGROUND

Aspects of the present disclosure relate generally to wirelesscommunication networks, and more particularly, to random access channel(RACH) procedure at a user equipment (UE).

Wireless communication networks are widely deployed to provide varioustypes of communication content such as voice, video, packet data,messaging, broadcast, and so on. These systems may be multiple-accesssystems capable of supporting communication with multiple users bysharing the available system resources (e.g., time, frequency, andpower). Examples of such multiple-access systems include code-divisionmultiple access (CDMA) systems, time-division multiple access (TDMA)systems, frequency-division multiple access (FDMA) systems, orthogonalfrequency-division multiple access (OFDMA) systems, and single-carrierfrequency division multiple access (SC-FDMA) systems.

These multiple access technologies have been adopted in varioustelecommunication standards to provide a common protocol that enablesdifferent wireless devices to communicate on a municipal, national,regional, and even global level. For example, a fifth generation (5G)wireless communications technology (which can be referred to as newradio (NR)) is envisaged to expand and support diverse usage scenariosand applications with respect to current mobile network generations. Inan aspect, 5G communications technology can include: enhanced mobilebroadband addressing human-centric use cases for access to multimediacontent, services and data; ultra-reliable-low latency communications(URLLC) with certain specifications for latency and reliability; andmassive machine type communications, which can allow a very large numberof connected devices and transmission of a relatively low volume ofnon-delay-sensitive information. As the demand for mobile broadbandaccess continues to increase, however, further improvements in NRcommunications technology and beyond may be desired.

5G/NR wireless systems target low latencies which need faster and moreefficient schemes for random access. However, the four-step randomaccess channel (RACH) procedure of LTE may not meet the low latencyrequirements of 5G/NR wireless systems. Therefore, a faster and moreefficient RACH procedure is desired.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed aspects will hereinafter be described in conjunction withthe appended drawings, provided to illustrate and not to limit thedisclosed aspects, wherein like designations denote like elements, andin which:

FIG. 1 is a schematic diagram of a wireless communication networkincluding at least one UE having a RACH component and at least one basestation having a corresponding RACH component, both RACH components areconfigured according to this disclosure for executing a RACH procedureat the UE.

FIG. 2A is a diagram illustrating an example of a DL frame structure.

FIG. 2B is a diagram illustrating an example of channels within the DLframe structure.

FIG. 2C is a diagram illustrating an example of an UL frame structure.

FIG. 2D is a diagram illustrating an example of channels within the ULframe structure.

FIG. 3 illustrates an example four-step RACH procedure at a UE.

FIG. 4 illustrates an example two-step RACH procedure at a UE accordingto an aspect of the present disclosure.

FIG. 5 is a flow diagram of an example method of a RACH procedures at auser equipment, according to an aspect of the present disclosure.

FIG. 6 is a schematic diagram of example components of the UE of FIG. 1.

FIG. 7 is a schematic diagram of example components of the base stationof FIG. 1.

SUMMARY

The following presents a simplified summary of one or more aspects inorder to provide a basic understanding of such aspects. This summary isnot an extensive overview of all contemplated aspects, and is intendedto neither identify key or critical elements of all aspects nordelineate the scope of any or all aspects. Its sole purpose is topresent some concepts of one or more aspects in a simplified form as aprelude to the more detailed description that is presented later.

According to one example, a method for a RACH procedure at a UE isprovided. The method includes selecting, at the UE, a two-step RACHprocedure or a four-step RACH procedure, wherein the selecting is basedat least on RACH configuration information received from a base stationor the RACH configuration information at the UE; and transmitting, fromthe UE, one or more messages associated with the two-step RACH procedureor the four-step RACH procedure based on the selection.

In another example, an apparatus for a RACH procedure at a UE isprovided. The apparatus includes a memory configured to store data; andone or more processors communicatively coupled with the memory, whereinthe one or more processors and the memory are configured to select, atthe UE, a two-step RACH procedure or a four-step RACH procedure, whereinthe selection is based at least on RACH configuration informationreceived from a base station or the RACH configuration information atthe UE; and transmit, from the UE, one or more messages associated withthe two-step RACH procedure or the four-step RACH procedure based on theselection.

In a further example, a user equipment for a RACH procedure is provided.The user equipment includes means for selecting, at the UE, a two-stepRACH procedure or a four-step RACH procedure, wherein the selecting isbased at least on RACH configuration information received from a basestation or the RACH configuration information at the UE; and means fortransmitting, from the UE, one or more messages associated with thetwo-step RACH procedure or the four-step RACH procedure based on theselection.

Additionally, in another example, a computer readable medium storingcomputer executable code for a RACH procedure at user equipment isprovided. The computer readable medium includes code for selecting, atthe UE, a two-step RACH procedure or a four-step RACH procedure, whereinthe selecting is based at least on RACH configuration informationreceived from a base station or the RACH configuration information atthe UE; and code transmitting, from the UE, one or more messagesassociated with the two-step RACH procedure or the four-step RACHprocedure based on the selection.

To the accomplishment of the foregoing and related ends, the one or moreaspects comprise the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrative featuresof the one or more aspects. These features are indicative, however, ofbut a few of the various ways in which the principles of various aspectsmay be employed, and this description is intended to include all suchaspects and their equivalents.

DETAILED DESCRIPTION

Various aspects are now described with reference to the drawings. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofone or more aspects. It may be evident, however, that such aspect(s) maybe practiced without these specific details. Additionally, the term“component” as used herein may be one of the parts that make up asystem, may be hardware, firmware, and/or software stored on acomputer-readable medium, and may be divided into other components.

The present disclosure generally relates to a RACH procedure at a UE(e.g., UE 110). For example, a base station (e.g., base station 105) maysend configuration information (e.g., RACH configuration information172, also referred to as configuration information) to the UE, e.g., UE110. The RACH configuration information indicates conditions that maytrigger selection of a two-step or a four-step RACH procedure at the UE.For example, based on the RACH configuration information received fromthe base station, the UE may determine RSRP values of a synchronizationsignal or a reference signal received from the base station and selectthe two-step RACH procedure if the RSRP value of the synchronizationsignal or the reference signal is equal to or above a threshold andselect the four-step RACH procedure if the RSRP value of thesynchronization signal or the reference signal is below the threshold.In another example, the conditions for triggering the two-step RACHprocedure may be defined in 3GPP Specifications. That is, there is noneed for the base station to send the RACH configuration information totrigger the two-step RACH procedure at the UE. The UE is to free tochoose the two-step RACH procedure as determined by the UE.

In the two-step RACH procedure, the UE collapses (e.g., combines) afirst message (e.g., message 1) and a third message (e.g., message 3) ofthe four-step RACH procedure into one message, e.g., message 13, andtransmits to the base station. The base station combines a second (e.g.,message 2) and a fourth message (e.g., message 4) of the four-step RACHprocedure and sends as a response (e.g., message 24) to the UE. Thecollapsing or combining of the messages provides a low-latency RACHprocedure at the UE. An optional sounding reference signal (SRS) whichmay be used as a reference signal is transmitted with the combinedmessage that is transmitted to the base station.

It should be noted that the techniques described herein may be used forvarious wireless communication networks such as CDMA, TDMA, FDMA, OFDMA,SC-FDMA, and other systems. The terms “system” and “network” are oftenused interchangeably. A CDMA system may implement a radio technologysuch as CDMA2000, Universal Terrestrial Radio Access (UTRA), etc.CDMA2000 covers IS-2000, IS-95, and IS-856 standards. IS-2000 Releases 0and A are commonly referred to as CDMA2000 1×, 1×, etc. IS-856 (TIA-856)is commonly referred to as CDMA2000 1×EV-DO, High Rate Packet Data(HRPD), etc. UTRA includes Wideband CDMA (WCDMA) and other variants ofCDMA. A TDMA system may implement a radio technology such as GlobalSystem for Mobile Communications (GSM). An OFDMA system may implement aradio technology such as Ultra Mobile Broadband (UMB), Evolved UTRA(E-UTRA), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20,Flash-OFDM™, etc. UTRA and E-UTRA are part of Universal MobileTelecommunication System (UMTS). 3GPP Long Term Evolution (LTE) andLTE-Advanced (LTE-A) are new releases of UMTS that use E-UTRA. UTRA,E-UTRA, UMTS, LTE, LTE-A, and GSM are described in documents from anorganization named “3rd Generation Partnership Project” (3GPP). CDMA2000and UMB are described in documents from an organization named “3rdGeneration Partnership Project 2” (3GPP2). The techniques describedherein may be used for the systems and radio technologies mentionedabove as well as other systems and radio technologies, includingcellular (e.g., LTE) communications over a shared radio frequencyspectrum band. The description below, however, describes an LTE/LTE-Asystem for purposes of example, and LTE terminology is used in much ofthe description below, although the techniques are applicable beyondLTE/LTE-A applications (e.g., to 5G networks or other next generationcommunication systems).

The following description provides examples, and is not limiting of thescope, applicability, or examples set forth in the claims. Changes maybe made in the function and arrangement of elements discussed withoutdeparting from the scope of the disclosure. Various examples may omit,substitute, or add various procedures or components as appropriate. Forinstance, the methods described may be performed in an order differentfrom that described, and various steps may be added, omitted, orcombined. Also, features described with respect to some examples may becombined in other examples.

Referring to FIG. 1, in accordance with various aspects of the presentdisclosure, an example wireless communication network 100 includes atleast one UE 110 with a modem 140 having a RACH component 150 thatmanages execution of a configuration information receiving component 152(optional), a selecting component 154, and/or a transmitting component156 for a RACH procedure at UE 110. The example wireless communicationnetwork 100 may further include a base station (or an eNB/gNB) 105 witha modem 160 and/or a corresponding RACH component 170 for transmittingRACH configuration information 172 to UE 110 and/or assist with the RACHprocedure at UE 110.

For example, UE 110 and/or RACH component 150 may be configured toreceive RACH configuration information 172 from base station 105. TheRACH configuration information 172 indicates to UE 110 the conditionsunder which UE 110 selects a four-step or a two-step RACH procedure. Thefour-step and the two-step RACH procedures are described below in detailwith respect to FIGS. 3 and 4, respectively.

In one aspect, UE 110 and/or RACH component 150 may include aconfiguration information receiving component 152 (optional) to receiveRACH configuration information 172, a selecting component 154 to selectthe two-step or the four-step RACH procedure, and/or a transmittingcomponent 156 to transmit one or more messages associated with thetwo-step or the four-step RACH procedure based on the selection. Basestation 105 may include a RACH component 170 to transmit RACHconfiguration information 172 to UE 110. In another aspect, theconditions for triggering the two-step RACH procedure may be defined in3GPP Specifications. That is, there is no need for the base station tosend the RACH configuration information to trigger the two-step RACHprocedure at the UE. The UE is to free to choose the two-step RACHprocedure as determined by the UE.

The wireless communication network 100 may include one or more basestations 105, one or more UEs 110, and a core network 115. The corenetwork 115 may provide user authentication, access authorization,tracking, internet protocol (IP) connectivity, and other access,routing, or mobility functions. The base stations 105 may interface withthe core network 115 through backhaul links 120 (e.g., S1, etc.). Thebase stations 105 may perform radio configuration and scheduling forcommunication with the UEs 110, or may operate under the control of abase station controller (not shown). In various examples, the basestations 105 may communicate, either directly or indirectly (e.g.,through core network 115), with one another over backhaul links 125(e.g., X1, etc.), which may be wired or wireless communication links.

The base stations 105 may wirelessly communicate with the UEs 110 viaone or more base station antennas. Each of the base stations 105 mayprovide communication coverage for a respective geographic coverage area130. In some examples, base station 105 may be referred to as a basetransceiver station, a radio base station, an access point, an accessnode, a radio transceiver, a gNB/NR for supporting 5G wirelesscommunications, a NodeB, eNodeB (eNB), Home NodeB, a Home eNodeB, arelay, or some other suitable terminology. The geographic coverage area130 for a base station 105 may be divided into sectors or cells makingup only a portion of the coverage area (not shown). The wirelesscommunication network 100 may include base stations 105 of differenttypes (e.g., macro base stations or small cell base stations, describedbelow). Additionally, the plurality of base stations 105 may operateaccording to different ones of a plurality of communication technologies(e.g., 5G (New Radio or “NR”), fourth generation (4G)/LTE, 3G, Wi-Fi,Bluetooth, etc.), and thus there may be overlapping geographic coverageareas 130 for different communication technologies.

In some examples, the wireless communication network 100 may be orinclude one or any combination of communication technologies, includinga NR or 5G technology, a Long Term Evolution (LTE) or LTE-Advanced(LTE-A) or MuLTEfire technology, a Wi-Fi technology, a Bluetoothtechnology, or any other long or short range wireless communicationtechnology. In LTE/LTE-A/MuLTEfire networks, the term evolved node B(eNB) may be generally used to describe the base stations 105, while theterm UE may be generally used to describe the UEs 110. The wirelesscommunication network 100 may be a heterogeneous technology network inwhich different types of base stations provide coverage for variousgeographical regions. For example, each base station or base station 105may provide communication coverage for a macro cell, a small cell, orother types of cell. The term “cell” is a 3GPP term that can be used todescribe a base station, a carrier or component carrier associated witha base station, or a coverage area (e.g., sector, etc.) of a carrier orbase station, depending on context.

A macro cell may generally cover a relatively large geographic area(e.g., several kilometers in radius) and may allow unrestricted accessby UEs 110 with service subscriptions with the network provider.

A small cell may include a relative lower transmit-powered base station,as compared with a macro cell, that may operate in the same or differentfrequency bands (e.g., licensed, unlicensed, etc.) as macro cells. Smallcells may include pico cells, femto cells, and micro cells according tovarious examples. A pico cell, for example, may cover a small geographicarea and may allow unrestricted access by UEs 110 with servicesubscriptions with the network provider. A femto cell may also cover asmall geographic area (e.g., a home) and may provide restricted accessand/or unrestricted access by UEs 110 having an association with thefemto cell (e.g., in the restricted access case, UEs 110 in a closedsubscriber group (CSG) of the base station 105, which may include UEs110 for users in the home, and the like). An base station for a macrocell may be referred to as a macro base station. An base station for asmall cell may be referred to as a small cell base station, a pico basestation, a femto base station, or a home base station. A base stationmay support one or multiple (e.g., two, three, four, and the like) cells(e.g., component carriers).

The communication networks that may accommodate some of the variousdisclosed examples may be packet-based networks that operate accordingto a layered protocol stack and data in the user plane may be based onthe IP. A user plane protocol stack (e.g., packet data convergenceprotocol (PDCP), radio link control (RLC), MAC, etc.), may performpacket segmentation and reassembly to communicate over logical channels.For example, a MAC layer may perform priority handling and multiplexingof logical channels into transport channels. The MAC layer may also usehybrid automatic repeat/request (HARQ) to provide retransmission at theMAC layer to improve link efficiency. In the control plane, the RRCprotocol layer may provide establishment, configuration, and maintenanceof an RRC connection between a UE 110 and the base stations 105. The RRCprotocol layer may also be used for core network 115 support of radiobearers for the user plane data. At the physical (PHY) layer, thetransport channels may be mapped to physical channels.

The UEs 110 may be dispersed throughout the wireless communicationnetwork 100, and each UE 110 may be stationary and/or mobile. A UE 110may also include or be referred to by those skilled in the art as amobile station, a subscriber station, a mobile unit, a subscriber unit,a wireless unit, a remote unit, a mobile device, a wireless device, awireless communications device, a remote device, a mobile subscriberstation, an access terminal, a mobile terminal, a wireless terminal, aremote terminal, a handset, a user agent, a mobile client, a client, orsome other suitable terminology. A UE 110 may be a cellular phone, asmart phone, a personal digital assistant (PDA), a wireless modem, awireless communication device, a handheld device, a tablet computer, alaptop computer, a cordless phone, a smart watch, a wireless local loop(WLL) station, an entertainment device, a vehicular component, acustomer premises equipment (CPE), or any device capable ofcommunicating in wireless communication network 100. Additionally, a UE110 may be Internet of Things (IoT) and/or machine-to-machine (M2M) typeof device, e.g., a low power, low data rate (relative to a wirelessphone, for example) type of device, that may in some aspects communicateinfrequently with wireless communication network 100 or other UEs 110. AUE 110 may be able to communicate with various types of base stations105 and network equipment including macro base stations, small cell basestations, macro gNBs, small cell gNBs, relay base stations, and thelike.

UE 110 may be configured to establish one or more wireless communicationlinks 135 with one or more base stations 105. The wireless communicationlinks 135 shown in wireless communication network 100 may carry uplink(UL) transmissions from a UE 110 to a base station 105, or downlink (DL)transmissions, from a base station 105 to a UE 110. The downlinktransmissions may also be called forward link transmissions while theuplink transmissions may also be called reverse link transmissions. Eachwireless communication link 135 may include one or more carriers, whereeach carrier may be a signal made up of multiple sub-carriers (e.g.,waveform signals of different frequencies) modulated according to thevarious radio technologies described above. Each modulated signal may besent on a different sub-carrier and may carry control information (e.g.,reference signals, control channels, etc.), overhead information, userdata, etc. In an aspect, the wireless communication links 135 maytransmit bi-directional communications using frequency division duplex(FDD) (e.g., using paired spectrum resources) or time division duplex(TDD) operation (e.g., using unpaired spectrum resources). Framestructures may be defined for FDD (e.g., frame structure type 1) and TDD(e.g., frame structure type 2). Moreover, in some aspects, the wirelesscommunication links 135 may represent one or more broadcast channels.

In some aspects of the wireless communication network 100, base stations105 or UEs 110 may include multiple antennas for employing antennadiversity schemes to improve communication quality and reliabilitybetween base stations 105 and UEs 110. Additionally or alternatively,base stations 105 or UEs 110 may employ multiple input multiple output(MIMO) techniques that may take advantage of multi-path environments totransmit multiple spatial layers carrying the same or different codeddata.

Wireless communication network 100 may support operation on multiplecells or carriers, a feature which may be referred to as carrieraggregation (CA) or multi-carrier operation. A carrier may also bereferred to as a component carrier (CC), a layer, a channel, etc. Theterms “carrier,” “component carrier,” “cell,” and “channel” may be usedinterchangeably herein. A UE 110 may be configured with multipledownlink CCs and one or more uplink CCs for carrier aggregation. Carrieraggregation may be used with both FDD and TDD component carriers. Thebase stations 105 and UEs 110 may use spectrum up to Y MHz (e.g., Y=5,10, 15, or 20 MHz) bandwidth per carrier allocated in a carrieraggregation of up to a total of Yx MHz (x=number of component carriers)used for transmission in each direction. The carriers may or may not beadjacent to each other. Allocation of carriers may be asymmetric withrespect to DL and UL (e.g., more or less carriers may be allocated forDL than for UL). The component carriers may include a primary componentcarrier and one or more secondary component carriers. A primarycomponent carrier may be referred to as a primary cell (PCell) and asecondary component carrier may be referred to as a secondary cell(SCell).

The wireless communications network 100 may further include basestations 105 operating according to Wi-Fi technology, e.g., Wi-Fi accesspoints, in communication with UEs 110 operating according to Wi-Fitechnology, e.g., Wi-Fi stations (STAs) via communication links in anunlicensed frequency spectrum (e.g., 5 GHz). When communicating in anunlicensed frequency spectrum, the STAs and AP may perform a clearchannel assessment (CCA) or a listen before talk (LBT) procedure priorto communicating in order to determine whether the channel is available.

Additionally, one or more of base stations 105 and/or UEs 110 mayoperate according to a NR or 5G technology referred to as millimeterwave (mmW or mmwave) technology. For example, mmW technology includestransmissions in mmW frequencies and/or near mmW frequencies. Extremelyhigh frequency (EHF) is part of the radio frequency (RF) in theelectromagnetic spectrum. EHF has a range of 30 GHz to 300 GHz and awavelength between 1 millimeter and 10 millimeters. Radio waves in thisband may be referred to as a millimeter wave. Near mmW may extend downto a frequency of 3 GHz with a wavelength of 100 millimeters. Forexample, the super high frequency (SHF) band extends between 3 GHz and30 GHz, and may also be referred to as centimeter wave. Communicationsusing the mmW and/or near mmW radio frequency band has extremely highpath loss and a short range. As such, base stations 105 and/or UEs 110operating according to the mmW technology may utilize beamforming intheir transmissions to compensate for the extremely high path loss andshort range.

FIG. 2A is a diagram 200 illustrating an example of a DL frame structureused for communications from base station 105 to UE 110. FIG. 2B is adiagram 230 illustrating an example of channels within the DL framestructure (e.g., downlink shared channel (DL-SCH)). FIG. 2C is a diagram250 illustrating an example of an UL frame structure used forcommunications from UE 110 to base station 105. FIG. 2D is a diagram 280illustrating an example of channels within the UL frame structure (e.g.,uplink shared channel (UL-SCH), physical random access channel (PRACH)).Other wireless communication technologies may have a different framestructure and/or different channels.

A frame (10 ms) may be divided into 10 equally sized subframes. Eachsubframe may include two consecutive time slots. A resource grid may beused to represent the two time slots, each time slot including one ormore time concurrent resource blocks (RBs) (also referred to as physicalRBs (PRBs)). The resource grid is divided into multiple resourceelements (REs). For a normal cyclic prefix, an RB may contain 12consecutive subcarriers in the frequency domain and 7 consecutivesymbols (for DL, OFDM symbols; for UL, SC-FDMA symbols) in the timedomain, for a total of 84 REs. For an extended cyclic prefix, an RB maycontain 12 consecutive subcarriers in the frequency domain and 6consecutive symbols in the time domain, for a total of 72 REs. Thenumber of bits carried by each RE depends on the modulation scheme.

As illustrated in FIG. 2A, some of the REs carry DL reference (pilot)signals (DL-RS) for channel estimation at the UE. The DL-RS may includecell-specific reference signals (CRS) (also sometimes called common RS),UE-specific reference signals (UE-RS), and channel state informationreference signals (CSI-RS). FIG. 2A illustrates CRS for antenna ports 0,1, 2, and 3 (indicated as R0, R1, R2, and R3, respectively), UE-RS forantenna port 5 (indicated as R5), and CSI-RS for antenna port 15(indicated as R).

FIG. 2B illustrates an example of various channels within a DL subframeof a frame. The physical control format indicator channel (PCFICH) iswithin symbol 0 of slot 0, and carries a control format indicator (CFI)that indicates whether the physical downlink control channel (PDCCH)occupies 1, 2, or 3 symbols (FIG. 2B illustrates a PDCCH that occupies 3symbols). The PDCCH carries downlink control information (DCI) withinone or more control channel elements (CCEs), each CCE including nine REgroups (REGs), each REG including four consecutive REs in an OFDMsymbol.

A UE may be configured with a UE-specific enhanced PDCCH (ePDCCH) thatalso carries DCI. The ePDCCH may have 2, 4, or 8 RB pairs (FIG. 2B showstwo RB pairs, each subset including one RB pair). The physical hybridautomatic repeat request (ARQ) (HARQ) indicator channel (PHICH) is alsowithin symbol 0 of slot 0 and carries the HARQ indicator (HI) thatindicates HARQ acknowledgement (ACK)/negative ACK (NACK) feedback basedon the physical uplink shared channel (PUSCH). The primarysynchronization channel (PSCH) may be within symbol 6 of slot 0 withinsubframes 0 and 5 of a frame. The PSCH carries a primary synchronizationsignal (PSS) that is used by a UE 104 to determine subframe/symboltiming and a physical layer identity. The secondary synchronizationchannel (SSCH) may be within symbol 5 of slot 0 within subframes 0 and 5of a frame. The SSCH carries a secondary synchronization signal (SSS)that is used by a UE to determine a physical layer cell identity groupnumber and radio frame timing. Based on the physical layer identity andthe physical layer cell identity group number, the UE can determine aphysical cell identifier (PCI). Based on the PCI, the UE can determinethe locations of the aforementioned DL-RS.

The physical broadcast channel (PBCH), which carries a masterinformation block (MIB), may be logically grouped with the PSCH and SSCHto form a synchronization signal (SS) block. The MIB provides a numberof RBs in the DL system bandwidth, a PHICH configuration, and a systemframe number (SFN). The physical downlink shared channel (PDSCH) carriesuser data, broadcast system information not transmitted through the PBCHsuch as system information blocks (SIBs), and paging messages.

As illustrated in FIG. 2C, some of the REs carry demodulation referencesignals (DM-RS) for channel estimation at the base station. The UE mayadditionally transmit sounding reference signals (SRS) in the lastsymbol of a subframe. The SRS may have a comb structure, and a UE maytransmit SRS on one of the combs. The SRS may be used by a base stationfor channel quality estimation to enable frequency-dependent schedulingon the UL.

FIG. 2D illustrates an example of various channels within an UL subframeof a frame. A physical random access channel (PRACH) may be within oneor more subframes within a frame based on the PRACH configuration. ThePRACH may include six consecutive RB pairs within a subframe. The PRACHallows the UE to perform initial system access and achieve ULsynchronization. A physical uplink control channel (PUCCH) may belocated on edges of the UL system bandwidth. The PUCCH carries uplinkcontrol information (UCI), such as scheduling requests, a channelquality indicator (CQI), a precoding matrix indicator (PMI), a rankindicator (RI), and HARQ ACK/NACK feedback. The PUSCH carries data, andmay additionally be used to carry a buffer status report (BSR), a powerheadroom report (PHR), and/or UCI.

FIG. 3 illustrates an example four-step RACH procedure 300 at a UE.

At operation 310, UE 110 and/or RACH component 150 may transmit (orsend) a message 1 (312) to base station 105. UE 110 may transmit message1 (312) using a preamble (also referred to as a RACH preamble, a PRACHpreamble, or a sequence, e.g., as shown in FIG. 2D) that is selectedfrom 64 RACH preambles or sequences. UE 110 also sends identity of UE110 to base station 105 so that the network (e.g., base station 105) canaddress UE 110 in the next operation (e.g., operation 320). The identityused by UE 110 may be a random access-radio network temporary identifier(RA-RNTI) which is determined from a time slot number in which the RACHpreamble or sequence is sent.

At operation 320, UE 110 and/or RACH component 150 may receive a message2 (322) from base station 105. UE 110 receives message 2 (322) inresponse to sending message 1 (312) to base station 105. Message 2 (322)may be a random access response (RAR) and received on a downlink-sharedchannel (DL-SCH) from base station 105. The RAR may be addressed to theRA-RNTI calculated by base station 105 from the timeslot in which thepreamble or sequence is sent. Message 2 (322) may also carry thefollowing information: a cell-radio network temporary identifier(C-RNTI) which may be used for further communications between UE 110 andbase station; a timing advance value which informs UE 110 to changetiming of UE 110 to compensate for round trip delay due to the distancebetween UE 110 and base station 105; and/or uplink grant resources whichmay be an initial resources assigned to UE 110 by base station so thatUE 110 can use a uplink-shared channel (UL-SCH) during operation 330, asdescribed below.

At operation 330, UE 110 and/or RACH component 150 may send message 3(332) to base station 105. UE 110 sends message 3 (332), which may be a“radio resource control (RRC) connection request message,” to basestation 105 in response to receiving message 2 (322) from base station105. The RRC connection request message may be sent to base station 105using the UL-SCH based on uplink grant resources granted duringoperation 320. UE 110 may use the C-RNTI that is assigned to UE 110 bybase station 105 during operation 320 when sending the RRC connectionrequest message.

Message 3 (332) or the RRC connection request message may include UEidentity, for example, a temporary mobile subscriber identity (TMSI) ora random value. The TMSI may be used for identifying UE 110 in a corenetwork (e.g., core network 115) and if UE 110 has previously connectedto the same core network (e.g., core network 115). Optionally, therandom value may be used if UE 110 is connecting to the network for thefirst time. Message 3 (332) may also include a connection establishmentcause which indicates the reason UE 110 needs to connect to the network(e.g., base station 105).

At operation 340, UE 110 and/or RACH component 150 may receive a message4 (342) from base station 105. Message 4 (342) may be a contentionresolution message from base station 105 if base station 105successfully received and/or decoded message 3 (332) sent from UE 110.base station 105 may send message 4 to base station 105 using the TMSIvalue of the random number described above, but may also contain a newC-RNTI which will be used for further communications between UE 110 andbase station 105. UE 110 uses the above described four-step RACHprocedure for synchronizing with the network when establishing aconnection.

FIG. 4 illustrates an example RACH procedure for NR 400 at a UEaccording to an aspect of the present disclosure.

At operation 410, UE 110 and/or RACH component 150 may transmit (orsend) a message 13 (412), also referred to a first message or a firstmessage of the two-step RACH procedure, to base station 105. In anaspect, for example, message 1 (312) and message (322) described abovein reference to FIG. 3 above, may be collapsed (e.g., combined) intomessage 13 (412) and sent to base station 105. Message 1 (412) mayinclude a sequence, which may have been selected from 64 possiblesequences, and may be used a reference signal (RS) for demodulation ofdata transmitted in message 13 (412).

At operation 420, UE 110 and/or RACH component 150 may receive a message24 (422), also referred to a second message or a second message of thetwo-step RACH procedure, from base station 105. UE 110 may receivemessage 24 (422) in response to sending message 13 (412) to base station105. Message 24 (422) may be a combination of message 2 (322) andmessage 4 (342) as described above in reference to FIG. 3.

The combining of messages 1 (312) and (332) into one message 13 (412)and receiving of message 24 (422) in response from base station 105allows the UE to reduce the RACH procedure setup time to support thelow-latency requirements of 5G/NR. Although, UE 110 may be configured tosupport the two-step RACH procedure, UE 110 still supports the four-stepRACH procedure as a fall back as the UE 110 may not be able to relay onthe two-step RACH procedure due to some constraints, e.g., high transmitpower requirements, etc. Therefore, a UE in 5G/NR may be configured tosupport both the two-step and the four-step RACH procedures, anddetermines which RACH procedure to configure based on the RACHconfiguration information received from base station 105.

FIG. 5 is a flowchart illustrating a method 500 for a RACH procedure ata UE.

In an aspect, at block 510, methodology 500 may include selecting, atthe UE, a two-step or a four-step RACH procedure, wherein the selectingis based at least on RACH configuration information received from a basestation or the RACH configuration information at the UE. For example, inan aspect, UE 110 and/or RACH component 150 may include selectingcomponent 154, such as a specially programmed processor module, or aprocessor executing specially programmed code stored in a memory, toselect the two-step or the four-step RACH procedure based at least onRACH configuration information 172 received from base station 105. TheRACH configuration information 172 indicates conditions that triggerselection of the two-step or the four-step RACH procedure. In additionalor optional aspect, the RACH configuration information 172 may bealready available (or present at UE 110) as defined, for example, by3GPP Specifications and/or UE 110 is free to choose either the two-stepor the four-step RACH procedure as the UE 110 see fit (as defined in the3GPP Specifications). That is, there is no need for UE 110 to receivethe RACH configuration information 172 from the base station 105, theRACH configuration information 172 may be already present/configured atUE 110.

In one aspect, for example, UE 110 may receive RACH configurationinformation 172 via a master information block (MIB) or a systeminformation block (SIB) broadcasted from base station 105. The MIB/SIBmay indicate conditions under which UE 110 may select a 2-step or a4-step RACH procedure. That is, the MIB/SIB may indicate conditions thattrigger selection of the two-step or the four-step RACH procedure. UE110 needs to receive (e.g., from base station 105) at least the MIB,SIB1, and/or SIB2 prior to initiating the RACH procedure at the UE. Forexample, the conditions that trigger selection of the two-step or thefour-step RACH procedure may be based on RSRP values of asynchronization signal or a reference signal received from base station105.

For instance, the RACH configuration information 172 may indicateconditions that trigger selection of the two-step or the four-step RACHprocedure. UE 110 may measure reference signal received power (RSRP) ofa synchronization channel or a reference signal received from basestation 105, compare the measured RSRP value with a threshold value,and/or select the two-step or four-step RACH procedure based on whetherthe RSRP value is above or below the threshold value. For example, UE110 may select the two-step RACH procedure if UE 110 measures the RSRPof the synchronization or the reference signal and determines that theRSRP value of the synchronization or the reference signal is equal to orabove the threshold value. In an additional or optional aspect, UE 110may select the four-step RACH procedure if the RSRP value of thesynchronization signal or the reference signal is below the thresholdvalue. The threshold value, for example, may be configured by basestation 105 and indicated to UE 110 via RACH configuration information172.

In an aspect, a low (e.g., lower than the threshold) RSRP at UE 110 mayindicate that UE 110 is located far (e.g., not near) from base station105 as compared to a UE with high (or higher) RSRP. That is, the RSRP ofthe synchronization signal or the reference signal at UE 110 is based(e.g., inversely related) on the distance from base station 105.Further, UE 110 may need higher transmit power for the two-step RACHprocedure (as compared to the four-step RACH procedure) as highertransmit power is needed to create a link (e.g., UL-SCH) with basestation 105. Furthermore, UE 110 may need higher transmit power totransmit Message 13 (412) using the two-step RACH procedure whencompared to transmitting a message 1 (312) using the four-step RACHprocedure as timing adjustment is not present in the two-step RACHprocedure. In other words, the transmit powers needed for transmittingmessage 13 (412) and message 1 (312) are different and higher fortransmitting message 13 (412) to base station 105. Moreover, thetransmit power required for initial access probe may be an offset ofRSRP, and the offsets may be different for message 1 (312) and message13 (412).

In an additional aspect, UE 110 may initiate the two-step RACH procedurebased on RSRP value (e.g., RSRP value equal or above the threshold) andmay switch to a four-step RACH procedure if the transmit power requiredto transmit message 13 (412) is high, or if the RSRP value falls belowthe threshold during re-transmission of message 13. Base station 105 maynotify UE 110 about the transmit power needed for configuring thetwo-step RACH procedure via RACH configuration information 172. Basestation 105 may also instruct the UE to switch from the two-step RACHprocedure to the four-step RACH procedure through its response toMessage 13. When UE 110 switches to the four-step RACH procedure aftertransmitting Message 13 and before receiving a response from basestation 105, UE 110 may continue using transmit power level that wouldhave been used if the switch had not occurred, or UE 110 may apply anoffset value to the power level, or UE may determine the power levelusing the method used to determine power level at the start of afour-step RACH procedure without applying any consideration to theprevious two-step RACH procedure transmissions. When UE 110 switches tothe four-step RACH procedure based on the response to Message 13received from base station 105, UE 110 may transmit Message 3 using apower level indicated in the response received from base station 105.

In an aspect, at block 520, methodology 500 may include transmitting,from the UE, one or more messages associated with the two-step or thefour-step RACH procedure based on the selection. For example, in anaspect, UE 110 and/or RACH component 150 may include transmittingcomponent 156, such as a specially programmed processor module, or aprocessor executing specially programmed code stored in a memory, totransmit, from UE 110, one or more messages based on the selection. Forinstance, UE 110 may transmit message 13 (422) if UE 110 selects thetwo-step RACH procedure and UE 110 may transmit message 1 (312), message3 (332) if UE 110 selects the four-step RACH procedure, and accordinglyreceive messages from base station 105.

For example, in an aspect, when UE 110 selects the two-step RACHprocedure, UE 110 may transmit message 13 (412) to base station 105.Message 13 (412) may be regular uplink data (e.g., control data) whichmay include a reference signal for demodulating UL data transmitted fromUE 110. In such a scenario, there is no need to separately send asequence, as described above in reference to FIG. 3, as the referencesignal transmitted with the UL data from UE 110 may be used as thesequence (e.g., replace the sequence). However, if UE 110 sends thesequence, the sequence may serve as the reference signal fordemodulating the uplink data transmitted from UE 110. If both thereference signal and the sequence are transmitted from UE 110, UE 110and/or RACH component 150 may combine the sequence and the referencesignal prior to transmitting to base station 105.

In one example, UE 110 may send a sounding reference signal (SRS) tobase station 105. The SRS is a reference signal transmitted by a UE andused by an base station for estimating uplink channel quality over awider bandwidth and uplink frequency selective scheduling. For example,in the two-step RACH configuration, UE 110 may transmit the SRS withmessage 13 (412) and base station 105 measures uplink quality. The SRSmay be sent on a same beam as a sequence and/or data, used as phasereference for data, and/or may be repeated to allow base station Rx beamtraining.

Further, the number of antenna ports used by UE 110 for transmitting theSRS may be different from the number of antenna ports used for PRACHtransmission. In an aspect, the maximum number of antenna ports that maybe used by the SRS may be signaled by base station 105 to UE 110 via theMIB or the SIBs, and/or the actual number of antenna ports used by UE110 for SRS transmission may be blindly detected at base station 105 orsignaled by UE 110 using different SRS resources. Furthermore, thebandwidth of the SRS may be different as compared to the referencesignal and/or data, and may signal UE bandwidth information. In anadditional or optional aspect, cyclic prefixes (CPs) of a referencesignal, data, and/or SRS need not be equal (e.g., may be different). ACP generally refers to prefixing of a symbol with a repetition of theend and may be signaled (e.g., broadcasted) by base station 105 to UEsvia a MIB and/or SIBs.

During a handover of UE 110 from one base station to another basestation (e.g., from a source base station to a target base station), aRACH (e.g., contention-free RACH) may also carry a payload instead ofjust a sequence (e.g., a PRACH sequence). The sequence may be replacedby or serve as a reference signal for the payload. The payload may be,for example, a measurement report, a buffer status report, channel statefeedback (CSF), and/or data. In one example, a handover message mayindicate timing adjust information, referred to as timing adjust, whichmay be estimated by the target base station based on detection of theSRS transmission from UE 110. The source base station may request suchSRS transmission which is transmitted with one or more directions by UE.In another example, the timing adjust information may be estimated bythe source base station based on deployment geometry, beam direction toa UE, and/or past history of timing adjust commands sent to the UE.Additionally, the source base station and the target base stationcommunicate with each other such that target base station is prepared toreceive the RACH.

Moreover, the waveform selection for the two-step RACH procedure, forexample, OFDM and SC-FDM may follow rules that are similar to message 3(332) of the four-step RACH procedure. For instance, the MIB and/or SIBsmay indicate different thresholds for the two-step and four-step RACHprocedures based on RSRP values. The thresholds, for example, may besemi-statically configured in MIB/SIBs. The MIB and/or SIBs may alsoindicate diversity scheme for message 13 (412) data and message 13 (412)may include beam-training signal request.

In an aspect, for example, in the four-step RACH procedure, a RACHsub-frame, which may be same or similar to sub-frames shown in FIGS.2A-2D, for example, may be reserved for message 1 (312) transmissionsthat may not have perfect timing adjustment, and the message 1 (312)transmissions under such conditions may be beam-paired withcorresponding downlink beams, e.g., a synchronization channel from thebase station. For the 2-step RACH procedure, the same beam pairing ismaintained, although, for larger UE transmissions (e.g., message 13(412) is larger than message 1 (312) as message 13 includes informationthat is in message 1 (312) and message 3 (332)). For example, separateRACH sub-frames may be used for message 1 (312) and message 13 (412),and the sub-frames optimized to the particular transmission. Forinstance, the sub-frame for message 13 (412) may have longer durationand/or different periodicity as compared to message 1 (312). However,the use of separate sub-frames is not optimal as it may involveadditional overhead due to the use of separate sub-frames.

In another aspect, beam-pairing may be maintained in the same sub-frame.However, this requires message 13 (412) and message 1 (312) have thesame transmission duration. This is possible by using a larger bandwidthfor message 13 (412) to accommodate the larger data payload of message13 (412). In a further additional aspect, message 13 (412) may betransmitted in two parts on two separate beams. The first part ofmessage 13 (412) may be similar to message 1 (312) and the second partof message 13 (412) may be similar to message 3 (332). The first and thesecond parts may include both reference signal and data transmissions,and the first part of message 13 (412) may carry information (e.g.,frequency assignment) of the second part as well. The reference signalsused in the two parts of message 13 (412) are related to each other by aone-to-one mapping so base station 105 could identify and match the twoparts.

In one aspect, for example, in case of a two beam message 13 (412),message 24 (422) may be received at UE 110 in two parts (or sent by basestation 105 in two parts) on two beams corresponding to the two beamsused for sending message 13 (412). In one example, the RAR payload maybe split between these two parts, or repeated across them with possiblydifferent redundancy versions (RVs) to allow soft combining across them.

In an additional aspect, in both the two-beam and one-beamconfigurations of message 13 (412), base station 105 may sometimesdetect the PRACH/reference signal sequence part (e.g., first part) butfail CRC-check on the data part (e.g., second part). In such a case, forexample, the RAR could indicate whether base station 105 successfullydecoded the second part of message 13 (412). For instance, the RAR maybe sent in user-specific search-space if successfully decoded and incommon search space if the decoding was not successful. Alternatively,the RAR may be sent as a message that is similar to message 2 (322) ofthe four-step RACH procedure instructing UE 110 to switch to thefour-step RACH message 3 (332) next. The base station 105 may then do aLLR-combining of the expected message 3 (332) transmission with dataportion of the initial message 13 (412) transmission.

Thus, UE 110 may execute a two-step RACH procedure, a four-step RACHprocedure at UE 110, and may transition from the two-step to thefour-step RACH procedure as needed to support URLLC in 5G/NR.

Referring to FIG. 6, one example of an implementation of UE 110 mayinclude a variety of components, some of which have already beendescribed above, but including components such as one or more processors612, memory 616, and transceiver 602 in communication via one or morebuses 644, which may operate in conjunction with modem 140 and RACHcomponent 150 to execute a RACH procedure at UE 110. Further, the one ormore processors 612, modem 140, memory 616, transceiver 602, RF frontend 688 and one or more antennas 665, may be configured to support voiceand/or data calls (simultaneously or non-simultaneously) in one or moreradio access technologies.

In an aspect, the one or more processors 612 can include a modem 140that uses one or more modem processors. The various functions related toRACH component 150 may be included in modem 140 and/or processors 612and, in an aspect, can be executed by a single processor, while in otheraspects, different ones of the functions may be executed by acombination of two or more different processors. For example, in anaspect, the one or more processors 612 may include any one or anycombination of a modem processor, or a baseband processor, or a digitalsignal processor, or a transmit processor, or a receiver processor, or atransceiver processor associated with transceiver 602. In other aspects,some of the features of the one or more processors 612 and/or modem 140associated with RACH component 150 may be performed by transceiver 602.

Also, memory 616 may be configured to store data used herein and/orlocal versions of applications 675 or RACH component 150 and/or one ormore of its subcomponents being executed by at least one processor 612.Memory 616 can include any type of computer-readable medium usable by acomputer or at least one processor 612, such as random access memory(RAM), read only memory (ROM), tapes, magnetic discs, optical discs,volatile memory, non-volatile memory, and any combination thereof. In anaspect, for example, memory 616 may be a non-transitorycomputer-readable storage medium that stores one or morecomputer-executable codes defining RACH component 150 and/or one or moreof its subcomponents, and/or data associated therewith, when UE 110 isoperating at least one processor 612 to execute uplink power controlcomponent 150 and/or one or more of its subcomponents.

Transceiver 602 may include at least one receiver 606 and at least onetransmitter 608. Receiver 606 may include hardware, firmware, and/orsoftware code executable by a processor for receiving data, the codecomprising instructions and being stored in a memory (e.g.,computer-readable medium). Receiver 606 may be, for example, a radiofrequency (RF) receiver. In an aspect, receiver 606 may receive signalstransmitted by at least one base station 105. Additionally, receiver 606may process such received signals, and also may obtain measurements ofthe signals, such as, but not limited to, Ec/Io, SNR, RSRP, RSSI, etc.Transmitter 608 may include hardware, firmware, and/or software codeexecutable by a processor for transmitting data, the code comprisinginstructions and being stored in a memory (e.g., computer-readablemedium). A suitable example of transmitter 608 may including, but is notlimited to, an RF transmitter.

Moreover, in an aspect, UE 110 may include RF front end 688, which mayoperate in communication with one or more antennas 665 and transceiver602 for receiving and transmitting radio transmissions, for example,wireless communications transmitted by at least one base station 105 orwireless transmissions transmitted by UE 110. RF front end 688 may beconnected to one or more antennas 665 and can include one or morelow-noise amplifiers (LNAs) 690, one or more switches 692, one or morepower amplifiers (PAs) 698, and one or more filters 696 for transmittingand receiving RF signals.

In an aspect, LNA 690 can amplify a received signal at a desired outputlevel. In an aspect, each LNA 690 may have a specified minimum andmaximum gain values. In an aspect, RF front end 688 may use one or moreswitches 692 to select a particular LNA 690 and its specified gain valuebased on a desired gain value for a particular application.

Further, for example, one or more PA(s) 698 may be used by RF front end688 to amplify a signal for an RF output at a desired output powerlevel. In an aspect, each PA 698 may have specified minimum and maximumgain values. In an aspect, RF front end 688 may use one or more switches692 to select a particular PA 698 and its specified gain value based ona desired gain value for a particular application.

Also, for example, one or more filters 696 can be used by RF front end688 to filter a received signal to obtain an input RF signal. Similarly,in an aspect, for example, a respective filter 696 can be used to filteran output from a respective PA 698 to produce an output signal fortransmission. In an aspect, each filter 696 can be connected to aspecific LNA 690 and/or PA 698. In an aspect, RF front end 688 can useone or more switches 692 to select a transmit or receive path using aspecified filter 696, LNA 690, and/or PA 698, based on a configurationas specified by transceiver 602 and/or processor 612.

As such, transceiver 602 may be configured to transmit and receivewireless signals through one or more antennas 665 via RF front end 688.In an aspect, transceiver may be tuned to operate at specifiedfrequencies such that UE 110 can communicate with, for example, one ormore base stations 105 or one or more cells associated with one or morebase stations 105. In an aspect, for example, modem 140 can configuretransceiver 602 to operate at a specified frequency and power levelbased on the UE configuration of the UE 110 and the communicationprotocol used by modem 140.

In an aspect, modem 140 can be a multiband-multimode modem, which canprocess digital data and communicate with transceiver 602 such that thedigital data is sent and received using transceiver 602. In an aspect,modem 140 can be multiband and be configured to support multiplefrequency bands for a specific communications protocol. In an aspect,modem 140 can be multimode and be configured to support multipleoperating networks and communications protocols. In an aspect, modem 140can control one or more components of UE 110 (e.g., RF front end 688,transceiver 602) to enable transmission and/or reception of signals fromthe network based on a specified modem configuration. In an aspect, themodem configuration can be based on the mode of the modem and thefrequency band in use. In another aspect, the modem configuration can bebased on UE configuration information associated with UE 110 as providedby the network during cell selection and/or cell reselection.

Referring to FIG. 7, one example of an implementation of base station105 may include a variety of components, some of which have already beendescribed above, but including components such as one or more processors712, memory 716, and transceiver 702 in communication via one or morebuses 744, which may operate in conjunction with modem 160 and RACHcomponent 170 to execute a RACH procedure at base station 105. Thecomponents in FIG. 7 that are similar to the components in FIG. 6 areconfigured to operate in a similar manner.

The above detailed description set forth above in connection with theappended drawings describes examples and does not represent the onlyexamples that may be implemented or that are within the scope of theclaims. The term “example,” when used in this description, means“serving as an example, instance, or illustration,” and not “preferred”or “advantageous over other examples.” The detailed description includesspecific details for the purpose of providing an understanding of thedescribed techniques. These techniques, however, may be practicedwithout these specific details. In some instances, well-known structuresand apparatuses are shown in block diagram form in order to avoidobscuring the concepts of the described examples.

Information and signals may be represented using any of a variety ofdifferent technologies and techniques. For example, data, instructions,commands, information, signals, bits, symbols, and chips that may bereferenced throughout the above description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, computer-executable code or instructionsstored on a computer-readable medium, or any combination thereof.

The various illustrative blocks and components described in connectionwith the disclosure herein may be implemented or performed with aspecially-programmed device, such as but not limited to a processor, adigital signal processor (DSP), an ASIC, a FPGA or other programmablelogic device, a discrete gate or transistor logic, a discrete hardwarecomponent, or any combination thereof designed to perform the functionsdescribed herein. A specially-programmed processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aspecially-programmed processor may also be implemented as a combinationof computing devices, e.g., a combination of a DSP and a microprocessor,multiple microprocessors, one or more microprocessors in conjunctionwith a DSP core, or any other such configuration.

The functions described herein may be implemented in hardware, softwareexecuted by a processor, firmware, or any combination thereof. Ifimplemented in software executed by a processor, the functions may bestored on or transmitted over as one or more instructions or code on anon-transitory computer-readable medium. Other examples andimplementations are within the scope and spirit of the disclosure andappended claims. For example, due to the nature of software, functionsdescribed above can be implemented using software executed by aspecially programmed processor, hardware, firmware, hardwiring, orcombinations of any of these. Features implementing functions may alsobe physically located at various positions, including being distributedsuch that portions of functions are implemented at different physicallocations. Also, as used herein, including in the claims, “or” as usedin a list of items prefaced by “at least one of” indicates a disjunctivelist such that, for example, a list of “at least one of A, B, or C”means A or B or C or AB or AC or BC or ABC (i.e., A and B and C).

Computer-readable media includes both computer storage media andcommunication media including any medium that facilitates transfer of acomputer program from one place to another. A storage medium may be anyavailable medium that can be accessed by a general purpose or specialpurpose computer. By way of example, and not limitation,computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, or any other medium that can be used to carry or store desiredprogram code means in the form of instructions or data structures andthat can be accessed by a general-purpose or special-purpose computer,or a general-purpose or special-purpose processor. Also, any connectionis properly termed a computer-readable medium. For example, if thesoftware is transmitted from a website, server, or other remote sourceusing a coaxial cable, fiber optic cable, twisted pair, digitalsubscriber line (DSL), or wireless technologies such as infrared, radio,and microwave, then the coaxial cable, fiber optic cable, twisted pair,DSL, or wireless technologies such as infrared, radio, and microwave areincluded in the definition of medium. Disk and disc, as used herein,include compact disc (CD), laser disc, optical disc, digital versatiledisc (DVD), floppy disk and Blu-ray disc where disks usually reproducedata magnetically, while discs reproduce data optically with lasers.Combinations of the above are also included within the scope ofcomputer-readable media.

The previous description of the disclosure is provided to enable aperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the common principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Furthermore, although elements of the describedaspects and/or embodiments may be described or claimed in the singular,the plural is contemplated unless limitation to the singular isexplicitly stated. Additionally, all or a portion of any aspect and/orembodiment may be utilized with all or a portion of any other aspectand/or embodiment, unless stated otherwise. Thus, the disclosure is notto be limited to the examples and designs described herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of a random access channel (RACH)procedure at a user equipment (UE), comprising: selecting, at the UE,between a two-step RACH procedure or a four-step RACH procedure, whereinthe selecting is based at least on RACH configuration informationreceived from a base station or the RACH configuration information atthe UE; and transmitting, from the UE, one or more messages associatedwith the two-step RACH procedure or the four-step RACH procedure basedon the selection.
 2. The method of claim 1, wherein the selectingincludes selecting the two-step RACH procedure, and wherein thetransmitting includes transmitting a first message of the one or moremessages, and wherein the first message includes a physical RACH (PRACH)sequence used as a reference signal (RS) by the base station or asounding reference signal (SRS).
 3. The method of claim 2, furthercomprising: transmitting, from the UE, a payload to the base stationduring a handover of the UE, wherein the payload is transmitted as partof the first message.
 4. The method of claim 3, wherein the payload is ameasurement report, a buffer status report, channel state feedback (CSF)information, or a user data.
 5. The method of claim 2, furthercomprising: receiving, from a base station during a handover of the UE,timing adjustment information.
 6. The method of claim 1, wherein theselecting includes selecting the two-step RACH procedure, and whereinthe transmitting further comprises: transmitting, from the UE, a firstmessage of the one or more messages to the base station; and receiving,from the a base station, a second message of the one or more messages,in response to transmitting the first message to the base station. 7.The method of claim 6, wherein the first message is transmitted over oneor more beams and the second message is received over the one or morebeams.
 8. The method of claim 7, wherein the second message is receivedover two beams of the one or more beams, and further comprising:receiving a random access response (RAR) payload in the second messageover the two beams, wherein the RAR payload is split over the two beamsor repeated over the two beams.
 9. The method of claim 1, wherein the UEreceives the RACH configuration information from the base station in amaster information (MIB) or a system information block (SIB).
 10. Themethod of claim 1, wherein the selecting includes selecting the two-stepRACH procedure, and further comprising: switching from the two-step tothe four-step RACH procedure based at least on a reference signalreceived power (RSRP) value of a synchronization signal or a referencesignal received from the base station, a transmit power, or a commandreceived from the base station.
 11. The method of claim 10, furthercomprising: using a transmit power based on the two-step RACH procedureprior to the switching, applying an offset to the transmit power basedon the two-step RACH procedure prior to the switching, or using atransmit power based on the four-step RACH procedure, and prior toreceiving a response from the base station.
 12. An apparatus for arandom access channel (RACH) procedure at a user equipment (UE),comprising: a memory configured to store data; and one or moreprocessors communicatively coupled with the memory, wherein the one ormore processors and the memory are configured to: select, at the UE,between a two-step RACH procedure or a four-step RACH procedure, whereinthe selecting is based at least on RACH configuration informationreceived from a base station or the RACH configuration information atthe UE; and transmit, from the UE, one or more messages associated withthe two-step RACH procedure or the four-step RACH procedure based on theselection.
 13. The apparatus of claim 12, wherein the one or moreprocessors and the memory are further configured to select the two-stepRACH procedure, and transmit a first message of the one or moremessages, and wherein the first message includes a physical RACH (PRACH)sequence used as a reference signal (RS) by the base station or asounding reference signal (SRS).
 14. The apparatus of claim 13, whereinthe one or more processors and the memory are further configured to:transmit, from the UE, a payload to the base station during a handoverof the UE, wherein the payload is transmitted as part of the firstmessage.
 15. The apparatus of claim 14, wherein the payload is ameasurement report, a buffer status report, channel state feedback (CSF)information, or a user data.
 16. The apparatus of claim 13, wherein theone or more processors and the memory are further configured to:receive, from a base station during a handover of the UE, timingadjustment information.
 17. The apparatus of claim 12, wherein theselection includes selecting the two-step RACH procedure, and whereinthe one or more processors and the memory are further configured to:transmit, from the UE, a first message of the one or more messages tothe base station; and receive, from the base station, a second messageof the one or more messages, in response to transmitting the firstmessage to the base station.
 18. The apparatus of claim 17, wherein thefirst message is transmitted over one or more beams and the secondmessage is received over the one or more beams.
 19. The apparatus ofclaim 18, wherein the second message is received over two beams of theone or more beams, wherein the one or more processors and the memory arefurther configured to: receive a random access response (RAR) payload inthe second message over the two beams, wherein the RAR payload is splitover the two beams or repeated over the two beams.
 20. The apparatus ofclaim 12, wherein the UE receives the RACH configuration informationfrom the base station in a master information (MIB) or a systeminformation block (SIB).
 21. The apparatus of claim 12, wherein theselection includes selecting the two-step RACH procedure, and whereinthe one or more processors and the memory are further configured to:switch from the two-step to the four-step RACH procedure based at leaston a reference signal received power (RSRP) value of a synchronizationsignal or a reference signal received from the base station, a transmitpower, or a command received from the base station.
 22. The apparatus ofclaim 21, wherein the one or more processors and the memory are furtherconfigured to: use a transmit power based on the two-step RACH procedureprior to the switching, apply an offset to the transmit power based onthe two-step RACH procedure prior to the switch, or use a transmit powerbased on the four-step RACH procedure, and prior to receiving a responsefrom the base station.
 23. An apparatus for a random access channel(RACH) procedure at a user equipment (UE), comprising: means forselecting, at the UE, between a two-step RACH procedure or a four-stepRACH procedure, wherein the selecting is based at least on RACHconfiguration information received from a base station or the RACHconfiguration information at the UE; and means for transmitting, fromthe UE, one or more messages associated with the two-step RACH procedureor the four-step RACH procedure based on the selection.
 24. Theapparatus of claim 23, wherein the selecting includes selecting thetwo-step RACH procedure, and wherein the transmitting includestransmitting a first message of the one or more messages, and whereinthe first message includes a physical RACH (PRACH) sequence used as areference signal (RS) by the base station or a sounding reference signal(SRS).
 25. The apparatus of claim 24, further comprising: means fortransmitting, from the UE, a payload to the base station during ahandover of the UE, wherein the payload is transmitted as part of thefirst message.
 26. The apparatus of claim 25, wherein the payload is ameasurement report, a buffer status report, channel state feedback (CSF)information, or a user data.
 27. The apparatus of claim 24, furthercomprising: means for receiving, from a base station during a handoverof the UE, timing adjustment information.
 28. The apparatus of claim 23,wherein the selecting includes selecting the two-step RACH procedure,and wherein the transmitting further comprises: means for transmitting,from the UE, a first message of the one or more messages to the basestation; and means for receiving, from the base station, a secondmessage of the one or more messages, in response to transmitting thefirst message to the base station.
 29. The apparatus of claim 28,wherein the first message is transmitted over one or more beams and thesecond message is received over the one or more beams.
 30. Anon-transitory computer readable medium storing computer executable codefor a random access channel (RACH) procedure at a user equipment (UE),comprising: code for selecting, at the UE, a two-step RACH procedure ora four-step RACH procedure, wherein the selecting is based at least onRACH configuration information received from a base station or the RACHconfiguration information at the UE; and code transmitting, from the UE,one or more messages associated with the two-step RACH procedure or thefour-step RACH procedure based on the selection.